In recent years, fuels for use in a diesel engine called biodiesel, which are produced from plant and/or animal fat and oil as raw materials, have been developed as part of countermeasures for suppressing global warming. In the process of producing the biodiesel, glycerol is produced as a side product in an amount of about 10% with respect to the oil as a raw material, and the effective use of the glycerol has been intensively studied.
On the other hand, propylene glycol, which is a compound in which a hydroxy group at the 1-position of the glycerol is converted to hydrogen, has been widely used in the fields of pharmaceuticals, cosmetics, foods and the like as a moisturizer, a lubricant, an emulsifier, an anti-freezing liquid, a solvent or the like, since this compound is less toxic to organisms as well as tasteless and odorless. In general, propylene glycol is produced by oxidizing propylene derived from petroleum called a fossil raw material to give propylene oxide, and then hydrating the same.
Regarding the effective use of glycerol, a method of converting glycerol to propylene glycol using a copper catalyst is known. Specifically, a method is disclosed in which glycerol is converted to propylene glycol and ethylene glycol by allowing the glycerol to react with hydrogen in the presence of a catalyst containing copper oxide and zinc oxide (see, for example, Patent Document 1).
The above disclosure merely discloses catalysts whose atomic ratio of copper to zinc (copper/zinc) is from 0.2 to 6, and details such as the conditions for producing the catalyst are not disclosed. In addition, due to the low catalytic activity, the yield of propylene glycol per unit volume and unit time of the catalyst-filled layer (hereinafter sometimes referred to as “space time yield”) is insufficient.
Further, a method is disclosed in which glycerol is converted to propylene glycol by allowing the glycerol to react with hydrogen in the presence of a catalyst containing copper and zinc (see, for example, Patent Document 2). However, details of the catalyst are not described.
Moreover, a method is disclosed in which glycerol is converted to propylene glycol by allowing the glycerol to react with hydrogen in the presence of a catalyst, which catalyst is obtained by allowing copper nitrate and zinc nitrate to react with urea, drying the reactant at 110° C. for 3 hours, and then calcining the same at a temperature of 300° C. for 3 hours (see, for example, Non-Patent Document 1).    Patent Document 1: U.S. Pat. No. 5,214,219    Patent Document 2: DE Patent Application Laid-Open Publication No. 4302464    Non-Patent Document 1: Catalysis Letters, Vol. 117, p. 62, 2007